1. Field of the Invention
The present invention relates to an image capturing apparatus and a control method therefor, and more particularly, to an image capturing apparatus which has a function of determining an expression of a subject, and a control method therefor.
2. Description of the Related Art
In a case in which a subject, in particular a person, is photographed using an image capturing apparatus such as a digital still camera, in addition to proper exposure and focusing it is desired that photographs that satisfy the person photographed, such as those with unclosed eyes and a bright facial expression, be taken.
In order to meet such requirements, Japanese Patent Laid-Open No. 2000-347277 discloses a camera for detecting changes of the eyes of a subject, such as blinks and the direction of line of sight, and carrying out shooting when predetermined shooting criteria are satisfied. Furthermore, Japanese Patent Laid-Open No. 2004-294498 discloses a system for carrying out automatic shooting by detecting a desirable expression or posing of a subject.
In recent years, with advances in semiconductor technology, the number of pixels of a sensor to be used for digital cameras and the like has exceeded ten million, thereby allowing images with higher resolutions to be captured. However, images with higher resolutions are not always captured, as images with lower resolutions are also captured depending on applications or settings. For example, images for recording moving images and images for displaying the appearance of subjects in electronic viewfinders in real time generally are given lower resolutions in order to reduce a processing load and the number of pixels that can be displayed on a display device.
Almost all of the pixels of an image sensor are read out in a case in which images with higher resolutions are to be obtained, whereas pixels are thinned out for reading or pixels are both thinned out and added for reading in a case in which images with lower resolutions are to be obtained.
Furthermore, images with lower resolutions can also have multiple resolutions, such as VGA moving images (640×480 pixels) and QVGA (320×240 pixels).
However, in a case in which changes in the state of the eyes of a person or expressions of the person are to be detected from images, the use of images with lower resolutions will result in lower detection rates because the images with lower resolutions produce less variations in the shape of the eyes or mouth than images with higher resolutions. Furthermore, in a case in which the face has a small ratio in images shot before thinning out pixels, the detection accuracy for the eyes and mouth is decreased in images with lower resolutions. Therefore, changes in the shape of the eyes or mouth will be even more difficult to detect, leading to failure to detect the change or an increase in the false detection rate.
If images with higher resolution are used to detect changes in the state of the eyes or to detect expressions, the false detection rate will likely be decreased. However, in a case in which images with higher resolutions are to be obtained, the reading time per image is increases compared to images with lower resolutions. As a result, the number of pixels that can be read per unit of time (frame rate) decreases. More specifically, the detection interval lengthens, decreasing trackability of the movements of the person. It is also possible to increase the operation speed of the image sensor to try to reduce the reading time for images with higher resolutions, but this increases power consumption.